Re: [PATCH v3 09/28] KVM: x86/mmu: Drop RCU after processing each root in MMU notifier hooks

[Mingwei Zhang <mizhang@google.com>]

On Sat, Feb 26, 2022, Sean Christopherson wrote:
> Drop RCU protection after processing each root when handling MMU notifier
> hooks that aren't the "unmap" path, i.e. aren't zapping.  Temporarily
> drop RCU to let RCU do its thing between roots, and to make it clear that
> there's no special behavior that relies on holding RCU across all roots.
> 
> Currently, the RCU protection is completely superficial, it's necessary
> only to make rcu_dereference() of SPTE pointers happy.  A future patch
> will rely on holding RCU as a proxy for vCPUs in the guest, e.g. to
> ensure shadow pages aren't freed before all vCPUs do a TLB flush (or
> rather, acknowledge the need for a flush), but in that case RCU needs to
> be held until the flush is complete if and only if the flush is needed
> because a shadow page may have been removed.  And except for the "unmap"
> path, MMU notifier events cannot remove SPs (don't toggle PRESENT bit,
> and can't change the PFN for a SP).
> 
> Signed-off-by: Sean Christopherson <seanjc@google.com>
> Reviewed-by: Ben Gardon <bgardon@google.com>

Reviewed-by: Mingwei Zhang <mizhang@google.com>
> ---
>  arch/x86/kvm/mmu/tdp_mmu.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/arch/x86/kvm/mmu/tdp_mmu.c b/arch/x86/kvm/mmu/tdp_mmu.c
> index 634a2838e117..4f460782a848 100644
> --- a/arch/x86/kvm/mmu/tdp_mmu.c
> +++ b/arch/x86/kvm/mmu/tdp_mmu.c
> @@ -1100,18 +1100,18 @@ static __always_inline bool kvm_tdp_mmu_handle_gfn(struct kvm *kvm,
>  	struct tdp_iter iter;
>  	bool ret = false;
>  
> -	rcu_read_lock();
> -
>  	/*
>  	 * Don't support rescheduling, none of the MMU notifiers that funnel
>  	 * into this helper allow blocking; it'd be dead, wasteful code.
>  	 */
>  	for_each_tdp_mmu_root(kvm, root, range->slot->as_id) {
> +		rcu_read_lock();
> +
>  		tdp_root_for_each_leaf_pte(iter, root, range->start, range->end)
>  			ret |= handler(kvm, &iter, range);
> -	}
>  
> -	rcu_read_unlock();
> +		rcu_read_unlock();
> +	}
>  
>  	return ret;
>  }
> -- 
> 2.35.1.574.g5d30c73bfb-goog
>